Direct Conversion Receivers (DCR) and Very Low Intermediate Frequency (VLIF) receivers are popular due to their relatively low cost and small size, as well as their ability to operate over a broad range of frequencies. These receivers combine a received radio frequency (RF) signal with a local oscillator signal prior to analogue to digital conversion.
In DCR, the local oscillator is set at the frequency of the RF signal to enable down converting of the RF signal directly into a baseband signal.
In VLIF receivers, the local oscillator is set at a frequency close to that of the RF signal, but differing by an offset that is referred to as an intermediate frequency. For example, the intermediate frequency is typically between 3.6 kHz to 5.7 kHz, depending on channel spacing. The local oscillator in the VLIF receiver thus operates at a frequency either just above or just below the frequency of the RF signal, i.e., on one “side” of the RF signal, and down converts the RF signal into a substantially baseband intermediate frequency signal.
In both DCR and VLIF receivers, the down converted RF signal comprises an in-phase component, I, and a quadrature component, Q. Often during down conversion, noise, distortion or imbalance is introduced into, or pronounced in, the I and Q components. For example, interferers, such as strong digital mobile radio (DMR) interferers, can generate undesired direct current (DC) transitions in the signal during down conversion. Such interference is known as chopper noise and can degrade the blocking performance of the receiver.
Accordingly, there is a need for improved interference detection and mitigation in RF receivers.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus (receiver) and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.